Erik,
A counter actually measures a number of phase measurements. Then, as you
process that you get a frequency readout based on the difference between
them (event-count divided by time between phase measurements). Now, as
you want to do frequency read-out, you can do a handful of filtering
mechanisms, and the CNT-91 can do the linear regression. This filtering
takes a number of samples and provides a filter to estimate frequency.
The consequence of that is that variations you see now have a different
scale than if you did the original calculation of only two
phase-samples. This creates a bias function and variations needs to be
corrected for to get numbers you can relate to the normal scale. It's
great for giving better frequency readings, but if you aim to quantify
the variations you end up fooling yourself.
Also, your assumption on observation frequency in Nyquist is wrong.
Turns out that aliasing of higher frequencies is very problematic. It is
only very recent instruments that can have the ability to avoid aliasing
(by using digital decimation), but a counter is not one of them, it is
fully exposed to the aliasing problem.
There is translations charts to convert the noise-amplitude of each
noise type into phase-noise and ADEV readings. If you have truely random
noise obeying the rules, you can convert between them. Toss in a spur,
and it works differently, and well, you need to convert those too
according to other rules. Look for "Enricos chart".
Noise types reaching for high frequencies compared to measurement tau0
will affect the resulting ADEV for sure. The bandwidth of that even
affects white phase modulation directly, and flicker phase modulation to
some degree.
So, a counter is really like an ADC for phase, with wide bandwidth input
and a sub-sampling mechanism (trigger/time-base). Through processing
frequency estimates can be provided. Aliasing occurrs in the
sub-sampling. Modern counters can provided estimation filters than goes
from a higher sub-sampling rate to a lower, which to some degree removes
aliasing, but not fully. These frequency estimation methods form a form
of decimation filter.
Cheers,
Magnus
On 2022-06-20 08:45, Erik Kaashoek via time-nuts wrote:
Bob,
Many thanks for the guidance you provide and the phase noise
measurement document.
Can you provide feedback on this reasoning: A counter is like an ADC
but in the frequency domain. So if you measure with 0.01 s tau you
basically average over 0.01 s so you can only observe "phase noise"
(e.g. energy that is not at the exact requested frequency) up to
maximum 50 Hz from the carrier. But as you measure the true frequency
changes the sensitivity of this measurement is extremely high.
Translating the amount of time spend at a certain frequency away from
the carrier (ADEV?) into a phase noise number in dBc is something I do
not yet understand.
With a (very good) spectrum analyzer you may be able to come close to
the carrier but as there is so much energy in the carrier it will be
difficult to observe phase noise energy closer than say 1 or 10 kHz
(at least not with the equipment I can afford) so any phase noise plot
created using a spectrum analyzer can not be better than the combined
phase noise of all LO's in the spectrum analyzer and will start at say
1 or 10 kHz.
For the frequencies between 50 Hz and 20 kHz the simplest option is to
use a second LO and a mixer and a slow (loop BW below 10 Hz)PLL to
keep the mixer in quadrature and feed the output of the mixer, after
low pass filtering, into a PC soundcard for FFT processing.
Erik.
On 19-6-2022 22:45, Bob kb8tq via time-nuts wrote:
Hi
As HP found out back around 1973 or so, translating ADEV to phase noise
is not possible. This is true, even if you have the ADEV numbers for
a variety
of Tau values as opposed to some sort of “average” kind of number.
There are a number of things ( like spurs ) that can strongly
influence a counter
based ADEV reading, and have very little impact on a phase noise ( or
signal to
noise reading. There also are ways the shape of the phase noise
curve can
impact ADEV and have very little signal to noise impact for a
specific signal.
By far the best way to do this is to properly measure phase noise at
various
offsets from carrier. You can then look at the dbc/Hz numbers at each
offset.
This lets you see what your devices are doing to the signal. You can
then track
down the offending bit or piece and fix the problem.
The easiest way I know of to do phase noise is to quadrature lock two
identical
sources into a double balanced mixer. You then put in a simple
amplifier stage
to drive the mix down output into a sound card or spectrum analyzer.
Total cost
if you already have a sound card should be < $50 ( US dollars …) for
a DIY version.
That assumes you have the usual junk box parts and do a point to
point wire
version.
Some example ADEV plots:
http://leapsecond.com/museum/manyadev.gif
<http://leapsecond.com/museum/manyadev.gif>
http://leapsecond.com/museum/manyadev.gif
<http://leapsecond.com/museum/manyadev.gif>
Some plots of a number of measurements:
http://www.leapsecond.com/pages/fe405/
<http://www.leapsecond.com/pages/fe405/>
Quick primer on phase noise measurement
https://www.npl.co.uk/special-pages/guides/gpg68_noise
<https://www.npl.co.uk/special-pages/guides/gpg68_noise>
( The easy approach starts on page 21 :) )
Bob
On Jun 19, 2022, at 11:40 AM, Karen Tadevosyan via time-nuts
<[email protected]> wrote:
Hi
Thank you for the clarification and rf-tools link.
Agree with your calculation. That’s why I raised this question
regarding a fixing PN degradation by Pendulum CNT-91.
Could you please explain where is the error in my reasoning of the
experiment :
* There is one 10 MHz OCXO with ADEV = 5 mHz
* There are two boards (DUT1 and DUT2) which multiply 10 MHz OCXO
signal by 2.5 using the PLL method
* DUT1 has 25 MHz output signal with high PN (checking by air
and by measurement of S/N)
* DUT2 has 25 MHz output signal with low PN (checking by air
and by measurement of S/N)
Experiment’s steps:
* Step 1: DUT1 ADEV measuring gives me a value of 60 - 70 mHz
instead of the expected 12.5 mHz (5 mHz x 2.5)
* Step 2: DUT2 ADEV measuring gives me a value of 10 - 12 mHz
which matches the expected 12.5 mHz (5 mHz x 2.5)
* Step 3: based on ADEV values which in the first case (DUT1) are
much greater than expected and in the second case (DUT2) coincide
with the expected I conclude that PN of the output signal from DUT2
will be lower than from DUT1.
I can see this PN degradation using Pendulum CNT-91 only as R&S FSQ8
does not fixate any PN degradation between DUT1 and DUT2
Karen, ra3apw
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